Fueling a Hot Debate on the Application of TiO2 Nanoparticles in Sunscreen

Abstract

Titanium is one of the most abundant elements in the earth's crust and while there are many examples of its bioactive properties and use by living organisms, there are few studies that have probed its biochemical reactivity in physiological environments. In the cosmetic industry, TiO₂ nanoparticles are widely used. They are often incorporated in sunscreens as inorganic physical sun blockers, taking advantage of their semiconducting property, which facilitates absorbing ultraviolet (UV) radiation. Sunscreens are formulated to protect human skin from the redox activity of the TiO₂ nanoparticles (NPs) and are mass-marketed as safe for people and the environment. By closely examining the biological use of TiO₂ and the influence of biomolecules on its stability and solubility, we reassess the reactivity of the material in the presence and absence of UV energy. We also consider the alarming impact that TiO₂ NP seepage into bodies of water can cause to the environment and aquatic life, and the effect that it can have on human skin and health, in general, especially if it penetrates into the human body and the bloodstream.

Keywords: nanoparticles; safety; skin; solubility; titanium dioxide; toxicity.

Figure 1

Environmental and biological effects of the application of TiO₂ nanoparticles (NPs) in sunscreen. The NPs can pollute water bodies and possibly hurt aquatic life, but also serve a beneficial photocatalytic sterilization function. In humans, the NPs may translocate into the body. There is evidence for proteins forming a protein corona around the NPs and influencing their cellular uptake. There are several UV and non-UV debilitating cellular effects caused by TiO₂ NPs. In both water bodies and humans, NP solubilization can occur, which produces Ti(IV) ions (not depicted) and effects similar to the NPs.

Figure 2

The semiconducting and photocatalytic properties of TiO₂ NPs.

Figure 3

Coordination similarities of siderophore binding of Ti(IV) and Fe(III) at pH 7.4. The hexadentate siderophores, Desferrioxamine B (DFOB) and Enterobactin, form 1:1 metal:ligand complexes. Citrate coordinates Ti(IV) and Fe(III) in slightly different ways. Citrate serves as a tridentate ligand when coordinated to Fe(III) and as a bidentate ligand when coordinated to Ti(IV).

Figure 4

Different pathways by which TiO₂ nanoparticles can enter and distribute in the human body.

Figure 5

The effect of TiO₂ NPs on infectious bacteria S. aureus in HeLa cells. The number of bacteria S. aureus in control vs. anatase and rutile exposed HeLa cells (a). TEM cross-sections of HeLa control cells (b), cells exposed to 0.1 mg/mL anatase (c), and 0.1 mg/mL rutile TiO2 (d) followed by exposure to S. aureus bacteria for 90 min. Arrows point towards bacteria. * Means p < 0.05. This figure was modified from Journal of Nanobiotechnology, 14:34, Copyright 2016, Springer Nature. This work was published under a CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/) [156].

Figure 6

Synchrotron X-ray fluorescence was used to identify and locate tattoo particle elements in skin and lymph nodes of a donor. (a) Visible light microscopy (VLM) images of the area were mapped by μ-XRF and tattoo pigments were indicated by a red arrow. (b) 4′,6-diamidino-2-phenylindole (DAPI) staining of the tissues showing the cell nuclei. (c) μ-XRF maps of P, Ti, Cl, and/or Br. For the lymph node, these areas are marked in (a,b). (d) Average μ-XRF spectra over the full area displayed in (c) * diffraction peak from sample support; ** scatter peak of the incoming beam. (e) Ti K-edge micro-X-ray absorption near edge structure (μ-XANES) spectra of skin and lymph node compared to the spectra of rutile, anatase, and an 80/20 rutile/anatase mixture calculation. This figure was obtained from Scientific Reports, 11395, Copyright 2017, Springer Nature. This work was published under a CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/) [166].